docs: def add(self, tag, ref):
          """Add to the vgroup an object identified by its tag and
          reference number.
  
          Args::
  
            tag       tag of the object to add
            ref       reference number of the object to add
  
          Returns::
  
            total number of objects in the vgroup after the addition
  
          C library equivalent : Vaddtagref
                                                """
  
          n = _C.Vaddtagref(self._id, tag, ref)
          _checkErr('addtagref', n, 'invalid arguments')
          return n